In conventional AC to DC power supply circuits, conversion is typically achieved by charging a capacitor input filter through a full-wave rectifying circuit. See, for example, Section 7.4 in James F. Gibbons' Semiconductor Electronics (McGraw-Hill 1966). In a particular type of AC to DC power supply, switches such as silicon controlled rectifiers or triacs are used as regulating switches that are turned on to allow energy to be transferred to the load. See, for example, "AC Phase Control", SCR Manual, Section 9 (5th Edition, General Electric Co. 1972) and Stephen Prigozy's "The Use of Transformer Leakage Inductance as a Parameter for Optimizing the Performance of SCR Power Supplies" at pp. 288-292 of the IEEE Journal of Solid-State Circuits, Vol. SC-7, No. 4 (August 1972).
There are several shortcomings in these circuits and in particular in the ones which utilize capacitor input filters. Current flow is limited to a very small angle in a cycle of applied AC voltage. These small conduction angles produce a high peak-to-average current ratio. This requires a trade-off between the ability of the power controller to produce a regulated power output with a desired ripple and the performance ratings of the switch and transformer and the size of the capacitor. Other shortcomings include the generation of radio frequency line interference, the need for auxiliary starting circuitry for the switches, the fact that the switches must carry both the load current and any overload current that may be developed without any inherent limitation on the current, and the fact that the circuitry responds only slowly to changes in line and load, typically taking more than one cycle to respond.